The present invention relates to a spark-ignition direct injection engine.
Arts for improving both an exhaust emission performance and a thermal efficiency have been known. For example, JP2007-154859A discloses such an art using a combustion mode of compression-igniting a gas mixture inside the cylinder. With engines where such compression-ignition combustion is performed, the compression-ignition combustion occurs with a significant increase in pressure as the engine load increases, causing increase of combustion noises. Thus, as disclosed in JP2007-154859A, even for the engines which perform compression-ignition combustion, within an operating range on a high engine load side, instead of the compression-ignition combustion, spark-ignition combustion by the operation of the ignition plug has generally been performed.
JP2009-197740A discloses an art of an engine which performs compression-ignition combustion within a low engine load operating range with low engine speed similarly to the engine disclosed in JP2007-154859A, in which open periods of intake and exhaust valves are adjusted to leave burned gas at a high temperature inside a cylinder within the compression-ignition combustion performing range so that the in-cylinder temperature is increased to stimulate the compression-ignition combustion, whereas the open timing of an intake valve is advanced within a part of the compression-ignition combustion performing range where the engine load is relatively high and the engine speed is relatively high so that burned gas inside the cylinder is blown back to the intake port side once and then the burned gas is introduced into the cylinder again along with fresh air. In this manner, the temperature of the burned gas decreases because of the fresh air. Thus, within the relatively high-engine-speed high-engine-load range where the temperature and pressure become higher at compression top dead center, sudden extreme pressure increases due to the compression-ignition combustion can be suppressed.
On the other hand, in spark-ignition combustion, since a thermal efficiency is relatively low, the burned gas temperature increases. In contrast, in compression-ignition combustion, as described in JP2007-154859A and JP2009-197740A, the high-temperature burned gas is introduced into the cylinder to secure the ignitability. Therefore, with engines where the combustion mode is switched as described in JP2007-154859A and JP2009-197740A, a temperature atmosphere inside the cylinder is comparatively high and the high-temperature burned gas produced by the spark-ignition combustion is introduced into the cylinder immediately after the spark-ignition combustion is switched to the compression-ignition combustion, resulting in excessive increases in the in-cylinder temperature. This excessive increase may cause such pre-ignition that the gas mixture within the cylinder is compressed to ignite in, for example, a compression-stroke period, and a pressure increase rate (dP/dt) inside the cylinder, in other words, a rate of a pressure change inside the cylinder (dp) per a predetermined period of time (dt) in a pressure increasing period, may become significantly high to cause loud combustion noises.
Note that, the switching from the spark-ignition combustion to the compression-ignition combustion is not limited to be performed according to the engine load decrease, but may also be performed while the engine load is stable, as well as when an engine temperature increases from a cold-start state to a warmed-up state.